In high-speed and high-density signal transmission between electronic devices or between wiring boards, signal transmission through conventional electric wiring suffers problems of inter-signal interference or signal attenuation, which limit the speed and density of signal transmission. In order to solve the limitation-related problem, studies have been conducted on a technique of optical connection between electronic devices or between wiring boards; i.e., optical inter-connection.
For transmitting an optical signal over a short distance within a device or between devices, a flexible film optical waveguide is required. Particularly when an optical waveguide is mounted in a small portable device, the optical waveguide is often mounted along the surface of a part of the device for saving space. Therefore, there is demand for a polymer-film optical waveguide which can be bent with a small radius of curvature.
In order to enhance the bending property of a flexible optical waveguide or the followability of the interface between layers during restoration of the optical waveguide, optical waveguides employing low-elastic-modulus material were previously developed. For example, Patent Document 1 discloses that the bending property of an optical waveguide is enhanced through use of a cladding layer thereof formed of an elastomer exhibiting a bending elastic modulus of 1,000 MPa or less after curing. Patent Document 2 discloses that a film optical waveguide in which cladding layers are joined by the mediation of a resin exhibiting a bending elastic modulus of 1,000 MPa or less and whose precursor has a hydrogen-bond group as a functional group exhibits enhanced followability of the interface between layers during bending or restoration of the optical waveguide. Particularly when a film optical waveguide which has upper and lower cladding layers each formed of an elastomer having a bending elastic modulus of 200 MPa and in which the cladding layer are joined by the mediation of a resin exhibiting a bending elastic modulus of 1,000 MPa or less and whose precursor has a hydrogen-bond group as a functional group is employed as a hinge of a mobile phone, bending with a radius of curvature as small as 1 mm is possible, and no peeling at the interface occurs after 200,000 repetitions of bending. However, since the optical waveguides disclosed in Patent Documents 1 and 2 are produced by means of a stamper, the freedom of design is limited, and change of the design is difficult, which are problematic. In Patent Documents 1 and 2, bending durability thereof is not studied in detail.
Patent Document 3 discloses a film-type optical waveguide which is produced from a photosensitive resin composition containing a (meth)acrylic polymer and a urethane (meth)acrylate compound and which exhibits a transmission loss of 0.5 dB/cm or less and excellent bending resistance. Regarding the (meth)acrylic polymer, Patent Document 3 discloses that when the weight average molecular weight exceeds 100,000, the viscosity of the composition increases, to impair coatability of the composition, which is problematic. In the Examples in Patent Document 3, (meth)acrylic polymers having a weight average molecular weight of 18,000 to 35,500 are employed, and bending resistance is evaluated by observing cracking or breakage of an optical waveguide when it is wound over a metallic rod having a radius of 2 mm. However, no description is provided on bending durability. When an optical waveguide is employed as a hinge of a mobile phone, twisting durability may be required in addition to bending durability. However, hitherto, no study has been conducted on twisting durability, and there has not been developed an optical waveguide exhibiting satisfactory bending durability and twisting durability.    Patent Document 1: Japanese Patent No. 3906870    Patent Document 2: Japanese Patent No. 3870976    Patent Document 3: Japanese Patent Application Laid-Open No. 2007-122023